POWER OF LITHARGE, ETC. 
271 
We know that the litharge of commerce should possess 
certain characteristics, which can be produced at will to a 
certain extent, by properly managing the duration of the 
time allowed for the cooling of the litharge that flows from 
the furnace. It is known that litharge when cooled quickly 
is yellow, or greenish yellow, and that litharge when slow- 
ly cooled, under the circumstances pointed out by M. Four, 
net, changes its structure and color, and acquires properties 
which cause it to be much sought after in commerce. 
It is to the study of the physical and chemical modifica- 
tions that produce this transformation, that I devoted my- 
self. 
M. Fournet admits, in effect, that litharge in fusion can 
obsorb oxygen, by becoming super-oxidised, and that at a 
higher temperature than is necessary for the decomposition 
of minium. This philosopher admits that the red litharge 
so much sought after in commerce owes its properties to an 
excess of oxygen. 
The opinion of M. Fournet is not shared by M.Thenard; 
this illustrious chemist repudiates the hypothesis of the 
super-oxidation of lead, at the temperature of the furnaces, 
cupel, and considers it possible that a solution of the oxy- 
gen in the litharge, analogous that of the same gas in melt- 
ed silver, takes place. Thus oxygen unites itself to the 
protoxide of lead when the cooling process is slow, and dis- 
engages itself when it is rapid. 
M. Pernolet, the present director of the mines and work- 
shops at Poultaouen and Huelgoat, had previously observed 
that litharge in fusion held a gas in solution, varying at 
the draught of air is employed, and the obstrichs, or black litharge, 
begins to pass over, it is a protoxide of lead mixed with oxysulphurets. 
The time occupied in the passing of the black litharge, varies accord- 
ing to the degree of the purity of the lead ; it may continue for six- 
teen or twenty ' hours before they begin to collect the litharge of com- 
merce. The total duration of the process for the refining of a ton of 
metal, is from 48 to 50 hours. 
